The present disclosure relates to an image reading apparatus and an image forming apparatus provided with the image reading apparatus and particularly relates to a technique for limiting a voltage level of an analog signal output from an image sensor.
Conventionally, a reading process for generating image data of a document facing a reading position has been performed in an image reading apparatus. In this reading process, a light source irradiates a beam of light (dotted line in FIG. 7) to a surface of a document placed on a contact glass and the beam of light is reflected, for example, as shown in FIG. 7. A part of reflected light (solid line in FIG. 7) is introduced to an image sensor such as a CCD via reflecting mirrors and a condenser lens. The image sensor converts the received light into an analog signal indicating the intensity of the received light by a voltage. The image reading apparatus repeatedly performs the above series of the reading process while moving the light source in a sub scanning direction or conveying a document in the sub scanning direction by an ADF (Auto Document Feeder). In this way, the image reading apparatus forms image data corresponding to the entire document and completes the reading of the document.
However, for example, as shown in FIG. 8, in the case of reading a glossy three-dimensional object such as a precious metal or a credit card as a document, light irradiated from a light source may be specularly reflected (specular reflection) by a document surface and the specularly reflected light may enter an image sensor such as because the glossy surface to be read is partly curved. This specularly reflected light may come to have intensity several times higher as compared with in the case of reading documents having a normal flat surface. This may cause a voltage level of an analog signal output from the image sensor to become several times higher than in normal times.
An AFE (Analog-Front-End) circuit is provided in a stage subsequent to the image sensor. The AFE circuit samples and amplifies the analog signal output from the image sensor on a pixel-to-pixel basis. A dynamic range of the input of the AFE circuit has become smaller with a tendency of recent years that a power-supply voltage of a circuit system decreases. Thus, if an analog signal having a high voltage level corresponding to specularly reflected light is input to the AFE circuit, a problem that the AFE circuit may erroneously operate to cause a defect in a read image.
To solve this problem, a technique for providing a clamp circuit and a limiter circuit between an image sensor and an AFE circuit has been proposed. The clamp circuit sets a black level (upper limit value) of an image signal output from the image sensor at such a level as not to exceed a limit value of the black level of the AFE circuit. The limiter circuit limits the level of the image signal output from the clamp circuit after the black level is set to such a level as not to exceed a limit value (lower limit value) of a white level of the input of the AFE circuit. By this, in the above technique, the level of the signal to be input to the AFE circuit can be limited within the dynamic range of the input of the AFE circuit.